BREAKING CHANGE: refactor(subscriber): remove unneeded code & utilize typescript

[callmehiphop]

Fixes #6
Fixes #12
Fixes #115
Fixes #119

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

---

Breaking change ahead!

There's been a small restructuring made to the subscription options.

Before:

const subscription = topic.subscription('my-sub', {
  batching: {
    maxMilliseconds: 100,
  },
  flowControl: {
    maxBytes: os.freem() * 0.2,
    maxMessages: 100,
  },
  maxConnections: 5,
});

After

const subscription = topic.subscription('my-sub', {
  ackDeadline: 10,
  batching: {
    callOptions: {}, // gax call options
    maxMessages: 3000,
    maxMilliseconds: 100,
  },
  flowControl: {
    allowExcessMessages: true,
    maxBytes: os.freem() * 0.2,
    maxExtension: Infinity,
    maxMessages: 100
  },
  streamingOptions: {
    highWaterMark: 0,
    maxStreams: 5, // formerly known as maxConnections
    timeout: 60000 * 5, // 5 minutes
  }
});

[JustinBeckwith]

Generally LGTM, but I don't have a great feel for the logic of the stuff you're replacing :)

[stephenplusplus]

Nice work! I really like the new hierarchy 👍

[JustinBeckwith]

👋 @callmehiphop I'm super excited for this to land :)

[callmehiphop]

@JustinBeckwith @stephenplusplus finished tests and did a few refactors here and there, but I think we're good to go!

[JustinBeckwith]

LGTM :) If @stephenplusplus is cool, I'm cool.

[callmehiphop]

I went ahead and made a couple perf updates. I think the one worth mentioning is the decision to bypass gax streams and use the grpc stream directly. With StreamingPull we need to buffer as few PullResponse objects as possible otherwise we run the risk of higher re-delivery rates. We will actually double the number of potential objects buffered by using gax since it wraps the grpc stream in a Duplex stream, adding another readable buffer to our pipeline. Currently gax does not offer any enhancements (retries, etc.) to BIDI streams, so this change should be a win/win scenario.